Spotting scopes are specialized optical telescopes used primarily for terrestrial observation, rather than astronomy, and which include a single optical path (monocular) and image-inverting optics. As compared to binoculars, spotting scopes typically have a higher optical power (usually magnifying between 12× and 60×), a narrower field of view, and a larger overall size and weight. While it is often desirable to have the increased optical power of a spotting scope, the relatively large size and weight of most known spotting scopes inhibits portability. The large size, relatively large mass, narrow field of view, and high optical magnification of such spotting scopes generally requires them to be supported on a tripod or other steady support for effective use. Moreover, most known spotting scopes have a limited operational focusing range that does not allow a user to focus on targets located closer than 30 feet (9.1 meters).
U.S. Pat. No. 4,669,833 of Mise illustrates in FIG. 2 a conventional focus adjustment mechanism used in spotting scopes, including a jack screw that extends through a housing of the scope. The jack screw has an axis of rotation aligned with an optical axis of the scope and is threadably engaged with a carriage that supports a prism inside the housing for linear movement along a pair of guide posts that are precisely aligned with the optical axis. A knob of the jack screw extends longitudinally from the housing and is manually rotated to drive the movable prism along the optical axis. A user rotates the jack screw knob by dragging a finger across the top of the knob or by reaching around the front of the knob to grasp it between the user's thumb and fingers—the latter motion being somewhat awkward. Rotation of the jack screw drives the carriage at a constant rate throughout the entire range of adjustment. The pitch of the jack screw threads is dictated by the fine adjustments required to achieve sharp focus as the focus setting approaches infinity. However, the present inventors have found that at closer focusing distances the effect of such fine adjustments are more subtle, which can make it difficult to quickly visually assess the effect of adjustments and determine the direction in which the jack screw should be turned to achieve sharp focus.
The present inventors have recognized a need for an improved spotting scope having a compact size, light weight, and an optical system that facilitates hand-held use, thereby eliminating or reducing the need for a tripod or other support device. A need has also been recognized by the present inventors for a spotting scope having an optical design that provides for improved close focus capabilities and a mechanical design that facilitates fast adjustment of the focus setting without compromising the user's ability to hold the spotting scope steady.